-
Faculty of Science and Technology
MASTERS THESIS
Study program/ Specialization:
Offshore Technology/ Marine and Subsea Technology
Spring semester, 2014
Open
Writer: Anders Simonsen
(Writers signature) Faculty supervisor: Eiliv Janssen
External supervisor(s): Charles Paton
Thesis title:
Inspection and monitoring techniques for un-bonded flexible
risers and pipelines
Credits (ECTS): 30
Key words: Flexible pipeline Flexible riser Inspection
Monitoring Failure modes Integrity management
Pages: 58
+ enclosure: 18
Stavanger, 16.06/2014 Date/year
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Abstract Un-bonded flexible pipelines and risers are an
alternative to conventional rigid steel pipes. The use of
flexible pipes has enabled development of several offshore
fields that seemed unfeasible with the
use of rigid pipes due to extensive seabed preparation and large
dynamic motions. The lack of
knowledge and integrity management tools for flexible pipes is a
limiting factor and cause pipelines
and risers to be replaced before their service life has been
reached. This thesis aims to discover the
critical failure modes for flexible pipes and explore
conventional and novel techniques for performing
inspection and monitoring. By understanding the failures and
having access to the necessary
technology the remaining lifetime and lifetime extension
calculation will be more accurate than to
date. Degradations and failure mechanisms will be detected at an
early stage giving the operators
better time to initiate mitigating and repair measures.
By contacting industry experts and performing a study of the
latest literature this thesis presents the
possible failure modes for flexible pipes, as well as a
screening of conventional and novel inspection
and monitoring techniques. Reported incidents for Norway, UK and
the rest of the world are
presented to reveal the greatest risks for flexible risers. An
integrity management strategy is based
on the reported incidents, including recommendations and purpose
of inspection and monitoring
techniques.
Based on incident reports and inputs from stakeholders the most
frequent and critical failures to
flexible pipes caused by to damage to and degradation of the
internal and external polymer sheath.
Breach of the outer sheath creates a hazardous environment in
the annulus leading to an increased
risk for several failure mechanisms. Degradation of the internal
pressure sheath threatens the fluid
containment integrity, and is difficult to inspect for. It is
recommended to have a well-functioning
annulus vent system attached to a monitoring system to control
the annulus environment.
Integrated fiber optics is considered as the most promising
technique currently under development.
This would provide continuous temperature monitoring throughout
the riser and pipeline which can
be used for outer sheath breach detection and temperature
degradation calculations. For existing
pipelines a number of solutions are under development for
integrity management, such as
radiography, ultrasonic testing, and magnetic stress measurement
among others. Individually they
are useful, but if combined they might act as powerful
multipurpose tools.
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Acknowledgments This thesis concludes my master degree in
Offshore Technology at the Faculty of Science and
Technology at the University of Stavanger. The work has been
carried out during the months January
through June 2014, in conjunction with A/S Norske Shell.
I would like to give my sincere gratitude to my supervisors,
Eiliv Janssen (UiS) and Charles Paton
(Shell), for their guidance and mentoring throughout the entire
process. They have given me the
motivation and encouragement to finish this thesis.
I would also like to thank my fellow students that have made my
days at the office to a positive
experience.
Anders Simonsen.
June 2014.
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IV
Table of contents Abstract
...................................................................................................................................................................
II
Acknowledgments
..................................................................................................................................................
III
List of figures
..........................................................................................................................................................
VI
List of tables
..........................................................................................................................................................
VII
Abbreviations
.......................................................................................................................................................
VIII
1 Introduction
...............................................................................................................................................
1
1.1 Background
............................................................................................................................................
1
1.2 Purpose of this thesis
............................................................................................................................
2
1.3 Limitations
.............................................................................................................................................
2
1.4 Methodology
.........................................................................................................................................
2
2 Flexible pipelines and risers
.......................................................................................................................
3
2.1 Introduction
...........................................................................................................................................
3
2.2 Layer description
...................................................................................................................................
3
2.2.1 Carcass
..........................................................................................................................................
4
2.2.2 Internal pressure sheath
...............................................................................................................
5
2.2.3 Pressure armour
...........................................................................................................................
5
2.2.4 Tensile armour
..............................................................................................................................
6
2.2.5 Outer sheath
.................................................................................................................................
6
2.2.6 Other layers
..................................................................................................................................
6
3 Failure mode review
..................................................................................................................................
8
3.1 Collapse
.................................................................................................................................................
8
3.2 Burst
....................................................................................................................................................
10
3.3 Tensile failure
......................................................................................................................................
12
3.4 Compressive failure
.............................................................................................................................
14
3.5 Overbending
........................................................................................................................................
15
3.6 Torsional failure
...................................................................................................................................
15
3.7 Fatigue failure
......................................................................................................................................
16
3.8 Erosion
.................................................................................................................................................
18
3.9 Corrosion
.............................................................................................................................................
19
3.10 Summary of failure modes
..................................................................................................................
20
4 Failure mechanisms
.................................................................................................................................
22
4.1 Polymer related failures
......................................................................................................................
22
4.2 Annulus environment
..........................................................................................................................
23
4.3 End fittings
...........................................................................................................................................
24
5 Reported failures
.....................................................................................................................................
25
6 Inspection techniques
..............................................................................................................................
27
6.1 Conventional inspection techniques
...................................................................................................
27
6.1.1 Visual inspection
.........................................................................................................................
27
6.1.2 Annulus pressure testing
............................................................................................................
28
6.1.3 Laser leak detection
....................................................................................................................
29
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V
6.1.4 Coupon testing
............................................................................................................................
29
6.2 New inspection techniques
.................................................................................................................
30
6.2.1 Radiography
................................................................................................................................
30
6.2.2 Ultrasonic testing
........................................................................................................................
32
6.2.3 Eddy
current................................................................................................................................
33
6.2.4 Magnetic stress measurement
...................................................................................................
35
6.3 Summary of inspection techniques
.....................................................................................................
36
7 Monitoring
techniques.............................................................................................................................
39
7.1 Conventional monitoring techniques
..................................................................................................
39
7.1.1 Annulus vent monitoring
............................................................................................................
39
7.1.2 Annulus vacuum testing
..............................................................................................................
39
7.2 New monitoring techniques
................................................................................................................
40
7.2.1 Ultrasonic testing
........................................................................................................................
40
7.2.2 Torsion monitoring
.....................................................................................................................
40
7.2.3 Fiber
optics..................................................................................................................................
41
7.2.4 Magnetic stress measurement
...................................................................................................
45
7.2.5 Acoustic emission
.......................................................................................................................
45
7.2.6 Vibration monitoring
..................................................................................................................
45
7.3 Summary of monitoring techniques
....................................................................................................
46
8 Discussion
................................................................................................................................................
48
8.1 Reflections
...........................................................................................................................................
48
8.2 Integrity Management Strategy
..........................................................................................................
49
8.2.1 Continuous monitoring
...............................................................................................................
49
8.2.2 Interval inspection
......................................................................................................................
50
8.2.3 Explorative inspection
.................................................................................................................
51
8.3 Futuristic view
.....................................................................................................................................
53
9 Conclusion
................................................................................................................................................
54
10 Future work
..............................................................................................................................................
55
11 References
...............................................................................................................................................
56
Appendix A Internal pressure sheath polymer characteristics
..............................................................................i
Appendix B Complete list of failure mechanisms
..................................................................................................
ii
Appendix C Failure modes on NCS (1995 2014), CODAM database.
................................................................
vii
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VI
1 List of figures Figure 1 - Floating production units with
flexible risers attached
.......................................................... 3
Figure 2 Example of a basic configuration of flexible pipes
................................................................
4
Figure 3 - Example of an interlocked carcass profile
..............................................................................
4
Figure 4 - Examples of pressure armour profiles
...................................................................................
6
Figure 5 - Carcass collapse
......................................................................................................................
9
Figure 6 - Rupture of external sheath due to leak in end fitting
.......................................................... 11
Figure 7 - Rupture of external sheath due to blocked vent tubes
....................................................... 11
Figure 8 - Tensile armour wire rupture due to abrasion
......................................................................
12
Figure 9 - Tensile armour wire rupture due to fatigue
.........................................................................
13
Figure 10 - Example of flexible riser configurations
.............................................................................
13
Figure 11 - Example of birdcaging
........................................................................................................
14
Figure 12 - Torsion at the top of a riser due to ruptured armour
wires .............................................. 16
Figure 13 - Internal erosion of carcass
.................................................................................................
18
Figure 14 - Corroded tensile armour wires due to breach of outer
sheath ......................................... 19
Figure 15 Example of internal pressure sheath cracking
...................................................................
22
Figure 16 Example of outer sheath cracking
.....................................................................................
23
Figure 17 - Example of end fitting configuration
..................................................................................
24
Figure 18 - Laser leak detection device - Smart Light Devices
.............................................................
29
Figure 19 - Coupon testing setup
.........................................................................................................
30
Figure 20 - Robot crawler with DR inspection tool
..............................................................................
31
Figure 21 - Radiographic image of a flexible riser
................................................................................
31
Figure 22 - Ultrasonic defect detection.
................................................................................................
32
Figure 23 - Result of ultrasonic inspection of an unflooded and
a flooded riser respectively ............. 33
Figure 24 - Basic eddy current theory
..................................................................................................
34
Figure 25 - Eddy current defect detecting
............................................................................................
34
Figure 26 - Electromagnetic modelling of MAPS probe and the
induced eddy current vectors .......... 35
Figure 27 - Results of the stress test done by MAPS prototype in
2011 .............................................. 36
Figure 28 - Torsion monitoring configuration and monitoring
software interface .............................. 41
Figure 29 - Torsion monitoring using magnetic sensors
......................................................................
41
Figure 30 - Fiber bragg grating
.............................................................................................................
42
Figure 31 - MODA system configuration
..............................................................................................
43
Figure 32 - External sheath wrinkling monitoring system
....................................................................
44
Figure 33 - Fiber optics integrated directly into the tensile
armour wires .......................................... 44
Figure 34 - Vibration monitoring laboratory test and field
configuration ........................................... 46
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VII
List of tables Table 1 - Different polymer materials used in
flexible pipes
..................................................................
5
Table 2 - Failure mode summary
...........................................................................................................
21
Table 3 - Reported failure of flexible risers in the Norwegian
Continental Shelf ................................. 25
Table 4 - Reported failure of flexible risers in Norway, UK and
worldwide ......................................... 26
Table 5 - Inspection techniques summary
............................................................................................
38
Table 6 - Monitoring techniques summary.
..........................................................................................
47
Table 7 - Internal pressure sheath polymer characteristics.
....................................................................
i
Table 8 - Potential pipe defects/failure mechanisms for static
applications. .......................................... v
Table 9 - Potential pipe defects/failure mechanisms for dynamic
applications. .................................... vi
Table 10 - Reported incidents on NCS (1995 - 2014), CODAM
database. ................................................ x
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VIII
Abbreviations
API
American Petroleum Institute
AUV
Autonomous Underwater Vehicles
CO2
Carbon dioxide
CODAM
COrrosion and DAMage database
CRA
Corrosion Resistant Alloy
CT
Computed Tomography
DP
Dynamic Positioning
DR
Digital Radiography
FAT
Fabrication Acceptance Test
FBG
Fiber Bragg Grating
FPSO
Floating Production, Storage and Offloading unit
GoM
Gulf of Mexico
H2S
Hydrogen sulphide
HDPE
High-Density Polyethylene
HIC
Hydrogen Induced Cracking
IMS
Integrity Management Strategy
JIP
Joint industry project
MBR
Maximum Bend Radius
MEC-FIT
Magnetic Eddy Current Flexible Riser Inspection Tool
NCS
Norwegian Continental Shelf
NDT
Non-destructive Testing
PA
Polyamide
PSA
Petroleum Safety Authorities
PVC
Polyvinyl Chloride
PVDF
Polyvinylidene fluoride
QA
Quality Assurance
QC
Quality control
R&D
Research and Development
ROV
Remotely Operated underwater Vehicle
RP
Recommended Practice
SLOFEC
Saturated Low Frequency Eddy Current
SSC
Sulphide Stress Cracking
TDP
Touch-down Point
TWI
The Welding Institute
UK
United Kingdom
UKCS
United Kingdom Continental Shelf
UT
Ultrasonic Testing
UTS
Ultimate Tensile Strength
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1 Introduction
1.1 Background
Flexible pipelines and risers have been in use worldwide for
over 30 years and serve as an alternative
for the conventional rigid steel pipe. Flexible un-bonded pipes
consist of different layers of different
materials that act together and allow for unique flexibility
compared to rigid steel pipes. This
complexity has facilitated the development of several fields and
subsea tiebacks that would have
been economically unfeasible with rigid pipes due to the need
for extensive seabed preparations.
Flexible risers ease the use of floating production facilities
since they have a better dynamic response
than conventional steel risers.
However, the complexity of flexible pipes makes them more
vulnerable to degradations and the
number of failure modes is considerably higher. The use of
different materials enclosed within
several layers makes the task of integrity management difficult.
Still after so many years of
development, manufacturing and operation of flexible pipes there
exists large knowledge and
technology gaps that need to be filled to meet integrity
requirements. The lack of knowledge and
appropriate technology has been the direct cause for a number of
flexible pipes to be taken out of
operation before their service life has ended. This is partially
caused of undetected defects or too
conservative remaining life predictions.
To mitigate the risk of failure and extend the service life of
flexible pipe the industry has to
cooperate. In recent years some initiatives have started and are
leading the way for the industry to
follow.
In 2009 Oil and Gas UK started a Joint Industry Project (JIP)
funded by international oil and gas
operators. The JIP was led by SureFlex and included
international oil and gas operators, a flexible
pipe manufacturer and a regulatory authority (ref. [19]). A key
objective for this project was to collect
data from international oil and gas companies regarding flexible
pipe use, degradation, and incidents.
In recent years the Norwegian Petroleum Safety Authority (PSA)
has motivated for focus on flexible
pipeline integrity. They manage the database COrrosion and
DAMage (CODAM) for reporting injuries
and incidents of offshore structures and pipeline systems on the
Norwegian Continental Shelf (NCS).
In addition to this, PSA publishes an annual report regarding
trends in risk level in the petroleum
activity (RNNP) for the NCS. In 2012 the report stressed the
concern of integrity management for
flexible pipes, and presented a list of improvement areas for
the industry (ref. [9, 33]):
The industry needs to address the following improvement
areas:
1. Updating standards with the most recent experience.
2. Integrity management of flexible risers with continuous
monitoring and systems for
documenting operations history, which are actively used in
follow-up.
3. Ensure good training and expertise throughout the
organization responsible for following up
integrity.
4. Clear and unambiguous responsibilities for safe operation and
integrity management.
5. The industry must do a better job at sharing information
between companies in order to
ensure continuous improvement throughout the sector.
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6. The industry must actively commit to research and development
in order to increase
knowledge about flexible risers.
7. Quick and precise incident reporting associated with
pipelines, risers and subsea facilities.
The two initiatives mentioned above are critical for the
industry to progress in solving the integrity
issues for flexible pipelines and risers. Information sharing is
key to get an overview and to update
standards and guidelines for safe fabrication and operation of
flexible pipeline systems.
1.2 Purpose of this thesis
This thesis will have three main objectives:
1. Present and describe the most probable failure modes for
flexible pipes including
occurrences and statistics.
2. Present and describe conventional and new techniques for
inspection and monitoring of
flexible pipes.
3. Propose an integrity management strategy for flexible pipes
considering the information
provided in the two previous objectives.
By describing the failure modes with occurrence and statistics,
the need for inspection and
monitoring techniques will be established. The presentation of
these techniques will serve as a
screening of available integrity management tools within the
industry. Conventional techniques are
not sufficient to fulfil the integrity requirements, hence new
technologies will be explored. Based on
objective one and two an integrity management strategy will be
proposed. The inspection and
monitoring techniques will be allocated into categories, and the
necessity of these techniques will be
discussed. These objectives will serve as guidance for point
one, two and six from PSAs list for
improvements regarding the flexible pipe area.
1.3 Limitations
This thesis will be limited to un-bonded flexible pipelines and
risers. Hence bonded flexible pipes will
not be discussed. Ancillary equipment, including spools and
jumpers, will be mentioned but, not
described in detail. The main focus will be directed on risers
as these are the subject of the largest
concern. Information on new technologies and failures on
existing developments are limited due to
competitive reasons.
1.4 Methodology
To establish an overview of the failure modes and mechanisms a
literature study will be performed.
Contact with PSA and the utilisation of their reports on this
subject, in combination with industry
standards, will give a basis for understanding the criticality
of the different failure modes and
mechanisms. Conference papers and contact with authors and
experts will provide information
regarding inspection and monitoring techniques. Through this
process a list of stakeholders will be
established and act as personal channels for information sharing
and input on important issues.
These inputs will provide different perspectives on the
integrity management issue to broaden the
understanding of the wider context.
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2 Flexible pipelines and risers
2.1 Introduction
A flexible pipe is a complex configuration of different flexible
layers (described in section 2.2) which
act together as one unit for containment of produced oil and gas
or injection of gas and water. The
flexible pipeline and riser have the same pipe configuration.
The flexible pipeline is installed on the
seabed and is not subjected to large and frequent movements,
hence it can be referred to as a static
application. Flexible risers are the connection between seabed
and production unit. The riser follow
the motions of the production unit, hence it can be referred to
as a dynamic application.
The use of flexible pipelines has the potential to severely
reduce the total installation cost compared
to rigid pipeline installation. For rough seabed routes a rigid
pipeline would require a large amount of
seabed preparation, meaning renting and deployment of costly
vessels and labour. An alternative is
to find another route resulting in a longer pipeline, more
material and fabrication cost and longer
installation time. Another advantage of flexible pipeline is the
change that may happen on the
seabed over time. Currents can move the sand from beneath the
pipeline creating free spans. This
can cause problems for rigid pipelines and more seabed
preparation may be needed.
Flexible risers have been a revelation for floating production
units. The dynamic motions of ships and
semisubmersibles can be relatively large, and in harsh
environments rigid pipelines can be
unfeasible. Figure 1 shows an illustration of a semisubmersible,
to the left, and a Floating Production,
Storage and Offloading unit (FPSO), to the right, both connected
to flexible risers.
Figure 1 - Floating production units with flexible risers
attached [sintef.no].
2.2 Layer description
Flexible pipelines are complicated compounds made from different
layers and materials, each with
its specific function. The main components of a flexible
pipeline are shown in Figure 2 and comprise:
- internal carcass
- internal pressure sheath
- pressure armour wires
- tensile armour wires
- external pressure sheath
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4
Operators can alter the configurations of their flexible pipes
to achieve the properties they desire for
each unique development. Examples include: adding layers to
reinforce, adjusting the angle of
armour wires to control the force distribution, vary materials
for the operational environment and
more. The different layers are described in the subsequent
sections.
Figure 2 Example of a basic configuration of flexible pipes
[28].
A section of the flexible pipe cross section that will be
frequently discussed in this thesis is the
annulus. The annulus is the free space between the internal
pressure sheath and the outer sheath,
e.g. the spacing between the steel wires. It is common practice
to install a vent system for the
annulus at the end fitting of the riser. This system includes
tubes leading from the annulus to vent
valves and is designed to release pressure and perform
monitoring and testing of the annulus
environment.
2.2.1 Carcass
The innermost layer of the flexible pipeline is called carcass.
The carcass is composed of stainless
steel strips in an interlocked profile (see Figure 3). The
carcass has two main functions; prevent
collapse due to hydrostatic pressure, and protect the other
layers from contact with the produced
fluids. The interlocked profile gives the carcass the ability to
flex in addition to preventing the
pipeline from collapsing.
Figure 3 - Example of an interlocked carcass profile [2].
The dimensioning factor for the carcass is the expected
hydrostatic. As the innermost layer the
carcass acts as an abrasion cover for the rest of the layers so
they dont come in contact with the
bore fluids. The produced fluids may contain solids with high
erosion threat. This have to be
considered and the steel type of the carcass must resist this
force so that the pipe does not fail due
to corrosion.
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5
2.2.2 Internal pressure sheath
The internal pressure sheath is an extruded polymer (plastic
material) layer that is designed to keep
the bore fluid integrity intact. The carcass is not leak proof,
so the internal pressure sheath is
required to prevent fluids from flowing freely into the pipe
annulus. There are different materials
with different benefits used for the internal pressure sheath.
Table 1 lists the different polymer
materials used in flexible pipes. For temperature limits, fluid
compatibility and blistering
characteristics see Appendix A.
Layer Material type
Internal pressure sheath HDPE, XLPE, PA, PVDF
Intermediate (anti-collapse) sheaths HDPE, XLPE. PA. PVDF,
TPE
Anti-wear layers PA, PVDF, HDPE
Outer sheath HDPE, PA, TPE
Insulation PP, PVC, PU Table 1 - Different polymer materials
used in flexible pipes [2].
The most commonly used polymers are HDPE and PA-11, as they have
a greater strain resistance,
lower cost, and there is a better understanding of how they
react over time compared to the
currently available other options.
However, the materials used for the pressure sheath is permeable
to low-molecular-weight
hydrocarbons, meaning there is a continuous diffusion of gasses
from the bore into the steel armour
layers. This was not a known phenomenon in the early years of
flexible pipe manufacturing, and has
caused problems later on (this will be discussed in chapter
three and four).
2.2.3 Pressure armour
The pressure armour (also known as the hoop stress armour)
serves to withstand the internal radial
pressure caused by the bore fluids and gases. This is an
interlocked metallic layer wound by profiled
wires (see Figure 4) with an angle close to 90, installed around
the internal pressure sheath. The
profile of the wires and angle of winding allows for flexibility
of the layer along with the mechanical
strength. If the pipe is to be subjected to a very high internal
pressure, a second armour layer may be
installed.
High strength carbon steel is commonly used in the pressure
armour. The strength of the material
used is based on design pressure and the sourness of the bore
content. The sourness of the bore
content is determined by the percentage of carbon dioxide (CO2)
and hydrogen sulphide (H2S). Sweet
and sour service is characterised by a low and high percentage
of CO2 and H2S, respectively. For
sweet service, high strength steel with Ultimate Tensile
Strength (UTS) of 1400MPa can be used. High
strength steel is prone to Hydrogen Induced Cracking (HIC) and
Sulphide Stress Cracking (SSC) which
is embrittlement of the steel caused by high values of H2S and
CO2, respectively (further explained in
section 3.7). Therefore sour service flexible pipes use steel
wires with UTS as low as 750MPa.
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6
Figure 4 - Examples of pressure armour profiles [2].
2.2.4 Tensile armour
The tensile armour layer is designed to withstand the axial
tension force in the pipe. Large tension
forces are experienced by the pipeline during installation. The
riser is under constant tension as it is
suspended from the production facility down to the seabed. The
tensile armour layer consists of
several steel wires wound in a helical pattern around the
pressure armour. The tensile armour is
always installed in pair layers with opposing helical pattern to
create torsional equilibrium in the
pipe. Carbon steel is commonly used, and the UTS is governed by
the same principals as for the
pressure armour, i.e. HIC and SSC along with design forces.
If large axial forces are expected additional layers of tensile
armour can be installed. The steel wires
are wound with an angle of 20-60 degrees from the longitudinal
axis. Lower angles are used when
the flexible pipe includes a pressure armour layer. If the
internal pressure is estimated to be low
there is no need for pressure armour. Then the angle is
typically 55 degrees to balance the radial and
longitudinal forces in the pipe.
2.2.5 Outer sheath
The external sheath can be made from the same extruded polymer
material as the internal sheath
(see Table 1), HDPE is often used. The outer sheath serves as an
abrasion layer and keeps the
seawater out of the annulus. This is a critical layer as outer
sheath damage is the most common
recorded failure mode (ref. [19]).
2.2.6 Other layers
The five layers discussed above are the main layers in a
flexible pipe configuration. As mentioned
there can be differences in these layers with regards to
material selection, angle of winding, number
of layers etc. To specify each individual flexible pipe for its
own development conditions and
purpose, there are also some other layers that can be added to
the cross section.
Anti-friction tape is a high strength tape that is wrapped
around the steel layers to prevent wear
when they are rubbing against each other in dynamic conditions
(installation, vessel motion,
environmental loads, etc.). High strength kevlar tape can be
added to make sure that the wires keep
their intended position and prevent wire buckling throughout the
installation and operational stages.
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7
For sour service developments with a high concentration of H2S a
double annulus solution has been
developed. In this concept the inner annulus contains the
pressure armour which does not require
high tensile strength steel, this allows for a relatively high
H2S concentration. A second polymer
sheath is wrapped around the pressure armour to make another
section of the annulus where the
tensile armour is located. Some H2S gas will be able to diffuse
through this layer as well, but not as
much as if there only was one polymer layer. This allows for
higher tensile strength in the tensile
armour without increasing the risk of SSC and HIC failures.
For developments with cold water it is possible to install an
insulation layer before the outer sheath.
This layer will preserve the heat of the produced fluids for
long distance pipelines. If the produced
fluids are cooled down to a certain temperature the risk of
hydrate and wax formation is increased.
These formations can accumulate and block the pipeline,
resulting in shut-down and potentially in
costly repairs.
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8
3 Failure mode review Because of the complex configuration of
the flexible pipeline with many layers and different
materials there are a great number of events that can cause
failure of the pipeline. This chapter will
discuss:
- The failure modes that are listed in API RP 17B (ref. [2])
- Some of the mechanisms that may lead to these failure
modes
- Occurrence of the failure modes in terms of examples and
statistics
- Design solutions that can prevent the failure modes from
happening
A failure mode is typically caused by a succession of defects or
degradation that leads to loss of
integrity. Integrity loss is characterised by loss of
containment, reduced production capability or
increased risk for human lives, environment or other assets. A
failure mechanism is a single defect or
degradation that individually does not cause the pipe to fail.
The complete list of potential failure
mechanisms for static and dynamic applications listed in [2] is
presented in Table 8 and 9 in Appendix
B.
The failure modes that will be discussed in this chapter
are:
1. Collapse
2. Burst
3. Tensile failure
4. Compressive failure
5. Overbending
6. Torsional failure
7. Fatigue failure
8. Erosion
9. Corrosion
3.1 Collapse
Collapse of a flexible pipeline is when the pressure sheath
and/or carcass fail by collapsing inwards to
cause severe problems for flow assurance and integrity. The
failure mechanisms that may lead to
collapse are different, but the common denominators are
excessive force or pressure, fabrication
anomalies, erosion, corrosion of carcass and transport and
installation damage.
Failure mechanisms
Collapse can be caused by excessive tension of the tensile
armour wires. The weight of a flexible riser
is relatively large; hence the tensile load near the hag-off
point on the riser is large, especially for
deep water developments. The tension causes the tensile wires to
tighten, due to the helical pattern,
and transforms the tension into compression on the layers within
and can cause collapse of the
carcass. Dynamic vessels and environmental forces will add to
total tension force.
Collapse due to excessive external pressure may have different
causes. Deepwater installations
experience large hydrostatic pressure, and if not constructed
correctly this may contribute to the
collapse. Example of manufacturing defects is high initial
ovality and high radial gap between
pressure armour and internal pressure sheath. Under operation
gasses will diffuse through the
carcass and internal pressure sheath into the annulus. If the
venting system is not working as
designed, due to flattened or blocked vent tubes, a residual
pressure will build inside the annulus and
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9
may cause the collapse. The most common reason for collapse in
the cases of trapped gas in the
annulus is when the pipe is subjected to rapid pressure release.
The speed of the depressurization
causes the pipe to be unable to stabilise the pressure so the
residual stress in the annulus cause the
carcass and/or internal pressure sheath to collapse.
Recent carcass collapse failures have shown that high pressure
pipes with extra pressure polymer
sheaths may cause diffused gas to be permanently or temporarily
trapped between the layers and
cause large radial inwards pressure high enough to cause
collapse (ref. [24]). Shrinking of the
polymer layers combined with self-weight is also known to cause
carcass collapse.
Transportation and installation is a critical phase for every
flexible pipe. With regards to collapse the
greatest dangers are dropped objects, collisions and other
unexpected forces that might cause
ovalisation, overbending and excessive tension of the pipe.
For production pipes there is a risk of sand in the produced
fluids. Especially for gas production pipes
this may cause erosion on the inside of the carcass. This will
in some places reduce the wall thickness
and also lead to local corrosion. Thinning of the carcass wall
reduces the collapse resistance.
Occurrence
The collected CODAM data presented in Table 3 shows that
collapse is the most common reported
incident for the NCS. In later years it has been discovered that
collapse is especially threatening for
double annulus risers. Statoil has had several incidents with
flexible risers designed with this
configuration. Problems with carcass collapse in flexible risers
at Njord A, Visund and Snorre B is
described in [21]. This investigation report reveals leakages
during shut down activity in three
different risers at three different location, all of the with
the same double annulus configuration.
Figure 5 presents an incident where the carcass has
collapsed.
Figure 5 - Carcass collapse [21].
A failure in a flexible water injection riser with a smooth bore
was experienced on the Draugen field
in January 2000 (ref. [25]). The flexible riser was installed in
a nitrogen pressurised J-tube. Nitrogen
diffused through the outer sheath and into the pipe annulus. At
some occasions the pressure in the
top of the riser became so low that it acted as a vacuum and the
pressure difference in the bore and
annulus exceeded the collapse pressure. This caused the inner
sheath to collapse. When reapplying
pressure to the bore the inner sheath was blown up again and the
injected water could flow freely.
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10
This happened several times until the inner pressure sheath
cracked and pressurised water filled the
annulus. Tension wires were then subjected to the full hoop
stress and started failing. When the
outer layers failed they slide down inside the J-tube causing
the internal pressure sheath to fail
immediately and in the end also the pressure armour after
substantial uncoiling.
Design solution
Increasing the thickness of the pressure armour will improve the
resistance against the external
hydrostatic pressure. Increasing the thickness and strength of
the internal pressure sheath and/or
carcass will improve the resistance against residual pressure in
the annulus.
Increasing the thickness of the different layers may have
limited effect if the transportation and
installation loads are large enough to damage the pipe.
Modification of transportation routines or
installation procedures could reduce the number of failures, not
only collapse, but also damage of
the outer sheath.
3.2 Burst
Opposite of collapse, burst is caused by internal pressure or
excessive forces and the materials will
rupture outwards.
Failure mechanisms
Two potential failure mechanisms leading to burst is rupture of
tensile or pressure armour due to
excessive internal pressure. These failures may be caused by
fault in engineering or fabrication of the
pipe, fault in pressure integrity modules upstream in the system
and operating above design limits.
Rupture of the external sheath is also failure mechanism that
may lead to loss of pipe integrity.
Diffused gasses in the annulus can cause pressure build up large
enough to rupture the external
sheath if the venting system is not working properly.
The probability of burst is largely increased by anomalies in
the flexible pipe layers. Fabrication
errors, internal corrosion and erosion or external abrasion will
decrease the burst resistance and
create weak spots.
Occurrence
The CODAM database includes several reported burst incidents of
flexible risers. Figure 6 shows an
example of burst of the external sheath near the end fitting. In
this case there was a leak across the
main seal inside the end fitting. This leak caused the pressure
to build up inside the annulus leading
to a rupture of the external sheath (ref. [11]).
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11
Figure 6 - Rupture of external sheath due to leak in end fitting
[11].
Figure 7 shows the rupture of the external sheath in a flexible
riser where the annulus vent system is
blocked or not functioning correctly. The diffused gas from the
bore fluid will over time increase the
pressure until it is larger than the burst resistance of the
external sheath. This is however a well-
known problem and large focus has in the later years been
directed to a well-functioning vent system
and vent gas monitoring.
Figure 7 - Rupture of external sheath due to blocked vent tubes
[1].
Design solutions
Also for burst failure mechanisms a design solution is to
increase the thickness of the pressure
armour or external sheath. Another solution is to increase the
material strength if feasible.
Design modification can dictate the distribution of force in the
different layers. Angle of the tensile
and pressure armour, shape of the wires and also installation
angle of the pipeline are things that can
prevent burst from occurring.
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12
Adding more armour layers will obviously increase the burst
resistance, but will also increase the size
and weight of the pipe. If this is a deepwater riser the tension
loads may be too large to make this
solution feasible.
3.3 Tensile failure
Tension forces are mainly a riser problem as it hangs from the
platform and all of the weight is
distributed to the top of the riser. But also static flowlines
can be subjected to excessive tensile
forces due to snagging by fishing trawl or ship anchors.
Failure mechanisms
Excessive tensile forces may cause rupture in the tensile
armour. The tensile armour wires are
designed to withstand loads well above the normal service loads,
but in case of corrosion, fabrication
anomalies or other factors the resistance may be reduced.
As the tensile wires are configured in a helical pattern tensile
force will tighten the wires and act as
compression on the layers within. This may lead to collapse of
the carcass and/or internal pressure
sheath.
Snagging of the pipe can also cause overbending in the pipe.
This may lead to rupture of tension
armour or buckling of the pipeline.
Occurrence
Tensile failure is not one of the most threatening failure modes
due to the high fatigue safety factor
and focus from the start of the flexible pipe era. In the
presented incident reports from CODAM
(Table 3) and SureFlex (Table 4) tensile failure represents a
low percentage of occurrences. However
tensile failure has occurred in flexible risers in Brazil due to
high tensile loads combined with
corrosion or fatigue (ref. [1]). In other words, failure due to
tension may happen if other factors like
corrosion, abrasion, collision, anomalies etc. alter the tensile
resistance of the flexible pipeline. The
deep water developments in the Gulf of Mexico (GoM), Brazil and
West Africa make these fields
more applicable to tensile failure due to the high axial loads
of the riser itself.
Figure 8 - Tensile armour wire rupture due to abrasion [14].
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13
Figure 8 shows how abrasion can wear down the external sheath
and some of the tensile wires
risking the entire pipeline to rupture. Figure 9 is an example
of how tensile wires can rupture due to
fatigue, this time near the end fitting.
Figure 9 - Tensile armour wire rupture due to fatigue [14].
Design solutions
To prevent tensile failure it is advised to increase the
thickness of the tensile armour wires or select a
higher strength material if feasible. Modifying the wire angle
alters the force distribution and can
contribute to better tension resistance. Modifying the
installation angle and pipeline route can
decrease the tension force. Ultimately adding more tension
armour layers should be considered.
Another solution is to bury the pipeline. This will protect the
pipe from snagging by fishing trawls and
ship anchors.
For risers the riser configuration (see Figure 10) can reduce
the tensile load experienced in the hang
off point. By installing floaters at a point on the riser to
create a lazy wave configuration some of the
tension is relieved for the hang-off point and distributed to
the arch over the floating buoy. This is
more common for deep water installations than for shallow water
as the tensile loads here are
significantly larger.
Figure 10 - Example of flexible riser configurations [2].
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14
3.4 Compressive failure
When a pipeline is installed the temperature of the pipeline
materials are equalised with the
temperature of the ambient water. After starting production,
warm gas and fluids conveyed in the
pipeline raises the temperature. This causes the materials in
the pipeline to expand and if it is
restricted by friction or constrained ends compression forces
will build and may cause buckling and
overbending.
Failure mechanisms
An experienced failure mechanism in static flowlines is radial
buckling, also known as bird-caging.
This phenomenon occurs when the compressive loads are large
enough to cause wire disordering.
Usually the external pressure acting on the outer sheath is
enough to prevent radial buckling to
occur, but when the outer sheath is damaged the buckling
resistance is severely reduced. This is
however not a common failure in newer because of the use of high
strength kevlar tapes used
around the armour wire layers to prevent failures like
birdcaging from happening.
Excessive compression force may also lead to upheaval buckling.
Upheaval buckling is when
compression forces press the pipeline towards a restricted end.
When it is not allowed to move the
pipeline arches upwards creating bending stress.
Occurrence
From SureFlex incident report we see that birdcaging is a
failure to be aware of, with 5% of the
reported incidents worldwide (2010) being categorised as bird
caging. Also overbending and
upheaval buckling are represented on the incident chart, both
being a result of compressive forces
(among other reasons). Figure 11 shows a small example of
birdcaging.
Figure 11 - Example of birdcaging [23].
Design solutions
To avoid bird-caging there are strengthening tapes developed for
giving the tensile armour layer
support and restricting them from buckling outwards. Also
thicker outer sheath will serve somewhat
of the same purpose.
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15
However the best solution is to avoid these large forces if it
is possible. Selecting a riser configuration
that does not cause restrictions, adding spools or pipeline
bends to safely guide the pipeline
expansion and burying the pipeline are ways to avoid large
compressive forces.
3.5 Overbending
Overbending is a threat especially for risers in at the
touch-down point (TDP) and for flowlines as a
result of buckling. Bending of a pipe cause compression on one
side and tension at the other that
may lead to collapse or rupture. Overbending can also be a
result of trawler or anchor snagging.
Failure mechanisms
For the carcass and internal pressure sheath overbending can
cause collapse due to the compression
force and also ovalisation that reduces the collapse resistance.
It can also be the reason for rupture
of the internal or external pressure sheath as result of the
tension forces.
Large bending forces may lead to unlocking of the interlocked
layers (carcass and pressure armour)
of the flexible pipe. This will reduce the collapse resistance
of the carcass and also the pressure and
tension resistance of both the carcass and the pressure
armour.
The outer sheath can suffer cracking if the bending stress is
large enough.
Occurrence
There has only been one reported incident regarding overbending
in the Norwegian sector. This
happened during installation over an arch in shallow water.
Other incidents may have happened, but
if the production has not started when the incident happens it
has probably not been reported. It is
known that the UK sector had several incidents during
installation in the 90s, of which both
overbending and birdcaging occurred. This was mostly due to poor
equipment and procedures,
reasons that are easy to correct.
Design solutions
Modifying the design of the flexible pipe, adding armour layers
and alternating the pipeline route can
prevent overbending. Rock dumping and burial of the pipeline can
prevent buckling that could have
led to overbending.
3.6 Torsional failure
As flexible risers are used for floating production systems they
are in constant motion due to wind,
waves and currents. This will sometimes cause torsion loads in
the riser that may lead to a failure
mode.
Failure mechanisms
As the tensile armour wires are configured in a helical pattern
they are subjected to tension or
compression as the riser is twisted. Excessive tension loads due
to twisting may lead to rupture of
one or several wires.
Torsional force in either direction on the flexible pipe may
cause problems. If the force is in the same
direction as the helical pattern of the wires, they will tighten
and collapse of the carcass and/or
internal pressure sheath may occur. If the torsional force acts
in the other direction the wires may be
subjected to excessive compression force causing radial buckling
or unlocking of the armour wires.
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16
Torsional force can also be caused by rupture of tensile armour
wires. If a number of wires were to
rupture the entire riser will twist to restore equilibrium (see
Figure 12). The torsional force created to
restore equilibrium may cause rupture of internal or external
pressure sheath, or increase
compressive force risking collapse of carcass and/or internal
pressure sheath.
Figure 12 - Torsion at the top of a riser due to ruptured armour
wires [14].
Occurrence
Torsional failure is not regarded as a large threat, and it is
not an incident that is often reported.
However it may contribute to other failures and one must be
aware of the possibility for torsional
failure.
Design solutions
Torsional loads can be reduced by modifying the system design.
By modifying the cross sectional
design the torsional capacity can be improved. Altering the wire
lay angle, adding armour layers or
strengthening tape around the armour layers are recommended
solutions to improve torsional
capacity.
3.7 Fatigue failure
In service the different layers of the flexible pipe will be
subjected to several different stresses. This
can be tension, compression, torsion, erosion, corrosion and
temperature variations. A single load
cycle of the mentioned stresses may not be large enough to
damage the pipe, but the accumulated
cycles can wear down the different layers.
Failure mechanisms
A known difficulty (especially in deep water developments) is
the fatigue failure of tension wires near
the end connection. Tension wires are designed to endure much
higher loads than they are subjected
to, but over time the tensile stress due to environmental forces
will accumulate and fatigue wire
rupture may occur.
Damage to the outer sheath is a well-known problem and is one of
the most occurring defects that
may lead to pipe failure. Abrasion against bell mouth, I-tube or
bend restrictor, interfacing
structures, other pipelines and the seabed at the TDP may lead
to damage and seawater ingress. The
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17
flooded annulus creates an environment that can rapidly decrease
the service life of the flexible pipe
due to corrosion of the tensile and pressure armour wires and
degradation of the polymer layers,
thus increasing fatigue damage.
The polymer layers of the pipe can also experience failure due
to time dependant stresses.
Temperature cycles and flooded annulus may cause the polymeric
layers to crack due to
embrittlement (described in section 4.1).
Friction forces between the different layers add to the wear and
tear and can increase the total
fatigue loading.
Two chemical reactions within the flexible pipeline that may
severely reduce the service life are SSC
and HIC. SSC is when the pipeline operates with a sour service
environment, e.g. there is a relatively
large amount of H2S in the produced fluids. The H2S reacts with
the steel layers if they are not rated
for sour service operation. The reaction will cause
embrittlement of the steel which reduces the
cracking resistance of the material. HIC is a phenomenon that
occurs when atomic hydrogen is
discharged and diffuses into the metal to cause embrittlement.
The discharging of atomic hydrogen
happens in the vicinity of the sacrificial anodes in the
cathodic protection system. Corrosion Resistant
Alloy (CRA) materials is the most susceptible to the atomic
hydrogen and therefor also in greater risk
of HIC.
Occurrence
Fatigue failure is a complicated subject, and numerous tests and
calculation models have been
developed to estimate the fatigue life of the armour wires,
aging of the polymeric sheaths, corrosion
and erosion rate of the carcass or tensile wires etc. With this
intense focus on fatigue and a
recommended safety factor of 10 on fatigue life the failures are
decreasing. In Norway there has not
been a fatigue failure of tensile wires (ref. [1]). In Brazil
and West Africa however there have been
some incidents due to fatigue failure in tensile armour, but
this is generally caused after corrosion or
sour service has severely decreased the fatigue life of the
wires (ref. [1]).
Fatigue failure may as discussed be rare without any other
influencing factors. Another of these
factors is fabrication faults. One example is explained in [1]
where the carcass is fabricated so that
the interlocked steel strips are unable to slide when they are
fully expanded or compressed causing
an unintended force distribution and fatigue damage to the
carcass. This kind of fatigue failure may
be one of a kind, but it can categorise as fabrication faults
which is a critical category for most
failures.
Design solutions
To mitigate the risk of fatigue failure it is recommended that
the system is designed to minimise the
fatigue loads. Optimizing the floater movement in the water
(mooring or Dynamic Positioning (DP)
system), install bend stiffeners to reduce the bending loads
near the riser hang off; optimise the riser
configuration (lazy wave, catenary, etc.) to minimise loads and
movement of the riser.
Reassess the materials used and the thickness and configurations
of these materials to ensure they
can cope with the loads, fluids and temperatures they will be
subjected to. Many new failure
mechanisms and operating conditions have been learned since the
first flexible risers were installed.
This has to be incorporated into the new design of flexible
pipes.
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18
Material selection is important to prevent SSC and HIC from
happening. Use steel that is rated for the
expected sourness of the produced fluids, and be aware that the
sourness can change over time.
3.8 Erosion
Erosion in flexible pipelines is when particles in the produced
fluids collide with the internal wall of
the carcass and over time causes thinning of the steel
layer.
Failure mechanisms
When solids (sand) are produced and conveyed through a
production pipeline erosion can be a
problem. Especially for gas production the solid fragments have
high velocity and collide with the
inner wall off the carcass. Some areas will be more endangered
by this problem as the lay route of
the pipe cause certain areas to experience more erosion than
others (bends and curves).
Development of hydrates in the flexible pipeline may also cause
erosion problems as this may
develop into solid material (ice) that can cause erosion if it
is broken loose and transported along the
pipe.
Erosion alone does not usually cause failure of the pipe, but
the erosion process wears down the
corrosion protecting layer of the internal carcass wall. If
erosion and corrosion act together the
thinning of the carcass may be sufficient enough to cause
collapse or rupture. Figure 13 shows an
example of internal erosion of a carcass.
Figure 13 - Internal erosion of carcass [1].
Occurrence
There has not been reported any flexible pipeline failures due
to erosion alone. The highest risk for
serious erosion is for gas production lines when sand is
produced with the high velocity gas.
Design solution
A good estimation of the sand content of the production volume
is essential to the design of the
carcass layer. Increased wall thickness or optimised material
selection can increase the erosion
resistance.
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19
To reduce the sand content of the produced fluids would be the
best solution to this problem. This is
often ensured by a filter in the well near the reservoir.
Optimising the lay route of the pipeline will ensure better flow
of the production fluid and decrease
the critical areas for erosion.
3.9 Corrosion
Corrosion of the steel layers is caused by a chemical reaction
in the material. This gradually destroys
the material and reduces resistance and fatigue life. A typical
reason for corrosion in pipelines is
when seawater saturated with oxygen comes in contact with the
steel layers to induce oxidation.
Corrosion is not by itself a normal cause to pipe failure, but
together with high loadings or fatigue
loads this is a serious threat of integrity. Figure 14 shows two
different cases of corrosion of the
tensile armour wires. Figure 14 a) and b) presents a fully
corroded single tensile armour wire, while c)
presents local corrosion on multiple tensile armour wires.
Figure 14 - Corroded tensile armour wires due to breach of outer
sheath [1].
Failure mechanisms
If the corrosion protection on the internal wall of the carcass
has eroded away, there might be
internal corrosion of the carcass.
Tensile or pressure armour wires are threatened by corrosion
both from produced fluids diffusing
through the internal pressure sheath, and seawater ingress due
to a damaged outer pressure sheath.
The greatest integrity risk has been experienced to be near the
splash zone near the top end of the
riser. If the outer sheath is damaged here causing seawater to
flood the annulus oxygen will be
continuously supplied and the corrosion process will be rapid
compared to if the same happened
some distance under water. The top section of the riser is not
protected by the cathodic protection
system (corrosion protection). Corrosion in the tensile and
armour wires will reduce their material
capacities and hence reduce the fatigue life and system
integrity.
Occurrence
The most frequent cause of corrosion is when there is a breach
of the outer sheath and seawater and
oxygen is allowed to flow into the annulus. A great deal of the
incidents leading to breach of the
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20
outer layer is related to transportation or installation of the
flexible pipeline where undiscovered
damages are not repaired, and abrasion due to contact with
ancillary equipment. All the latest
statistics reveals this problem hence it is reason to believe
that this is of high concern and that the
occurrence of this type of failure will decrease in the coming
years. Especially the transportation and
installation process can be improved, and should not cause a
threat to the integrity.
Design solutions
To improve corrosion resistance increased layer thickness and
critical material selection is advised.
By adding coatings and/or lubricants the surface of the steel
layers will have larger scratch
resistance hindering local corrosion attacks. Coating on the
outer sheath will protect against
abrasion. Lubrication in the annulus protects the armour wires
from frictional damage.
Cathodic protection is a normal technique used for preventing
corrosion in pipelines. This is achieved
by installing sacrificial anodes of a metal that corrodes
instead of the pipeline. These anodes need to
be periodically replaced.
3.10 Summary of failure modes
Table 4 gives a summary of the failure modes listed and
described in this chapter. The table includes
a brief description of failure mechanisms, occurrence and design
solution for each failure mode.
# Failure mode Failure mechanisms Occurrence Design
solutions
1 Collapse
Excessive tension Large problem, multiple reports
both in CODAM and SureFlex JIP.
Problem worldwide
Increased thickness of pressure
armour/carcass
External pressure Increased thickness and
strength of internal pressure
sheath Residual pressure in annulus
Fabrication, transportation,
installation error
QA/QC in fabrication,
transportation and installation
Aging of polymer (shrinking)
Ovalisation
2 Burst
Rupture of tensile armour wires Burst of the outer sheath is
a
common problem. Rupture of
tensile wires may be a problem
for deepwater developments.
Increase thickness of tensile
armour wires
Rupture of pressure armour wires Increase thickness of
pressure
armour wires
Residual pressure in annulus Alter design configuration
3 Tensile failure
Excessive tensile force Not a frequent failure mode.
High risk for corroded wires in
deep water developments
Increase thickness of tensile
armour wires
Large dynamic movement Material selection
Corrosion combined with high
tensile loads
Alter design configuration
Riser configuration
Bury pipeline
4 Compressive
failure
Radial buckling Radial buckling (birdcaging) has
been reported several times
worldwide
Add strengthening tapes
Upheaval buckling Increase thickness of outer
sheath
Riser and pipeline
configuration
Bury pipeline
5 Overbending
Excessive bending force Problem at end of pipelines and
TDP for risers. Several
occurrences due to sloppiness in
the 90's
Riser and pipeline
configuration
Installation error Bury pipeline
Ancillary equipment Alter design configuration
6 Torsional
failure
Large dynamic movement Not a frequent failure mode.
Risers in harsh weather
conditions are most vulnerable
Riser configuration
Large environmental forces Alter design configuration
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21
# Failure mode Failure mechanisms Occurrence Design
solutions
Rupture of tensile armour wires Adding strengthening tape or
armour layers
7 Fatigue failure
Rupture of tensile armour wires Fatigue alone is not the
most
occurring failure mode due to a
very high safety factor. In
combination with erosion,
corrosion and other factors the
fatigue life is severely reduced.
Riser and pipeline
configuration
Rupture of pressure armour wires Material selection
Aging of polymer layers QA/QC in fabrication,
transportation and installation Cracking of carcass or armour
wires
8 Erosion
Internal erosion of carcass No reported failures. Risk when
sand bore fluids contain sand,
especially in high velocity gas
pipelines
Limit sand production
Increase carcass thickness
Pipeline route configuration
9 Corrosion
Rupture of tensile armour wires Large problem linked to the
frequent damage of outer
sheath.
Increase steel layer thickness
Rupture of pressure armour wires Add coatings or lubricants
Corrosion of internal carcass Material selection
Cathodic protection
Table 2 - Failure mode summary
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22
4 Failure mechanisms In addition to the failure modes described
in chapter three, there are some common failure
mechanisms that should be explained further to get a clear
overview of the risks involved with
operation of flexible pipelines. This chapter includes typical
failure mechanisms for the polymer
sheaths, how the annulus environment can cause degradation to
different layers and failures that
can happen in the end fittings.
4.1 Polymer related failures
The use of polymer sheaths in flexible pipelines is one of the
elements that make this technology
possible. Without these materials the pipeline would not be able
to contain the produced
hydrocarbons or keep the seawater out of the pipe annulus.
However the materials used have very
different properties compared to steel. One of the consequences
of the different properties is the
increase or decrease in volume. For example the expansion
coefficient for some polymer materials
are more than 10 times higher than steel. For rapid cool down of
pipelines, i.e. installation and shut-
down scenarios, this causes the polymer sheaths to reduce in
volume much faster than steel. This
can result in large stresses in the pressure sheath that can
cause cracks or transfer the loads to the
carcass, and in worst case cause carcass collapse or end fitting
pull-out.
Figure 15 Example of internal pressure sheath cracking [1].
To give the polymer layers its flexibility they contain an
additive called plasticiser. When in contact
with production fluids this additive will migrate out of the
polymeric sheath causing it to reduce in
volume and become more brittle. Figure 15 shows the cracking of
the internal pressure sheath.
Figure 16 shows cracking of the outer sheath.
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23
Figure 16 Example of outer sheath cracking [1].
Time dependant degradation is a frequent problem for the polymer
layers. Exposure to water and
high temperature can cause hydrolysis which alters the molecular
build-up of the polymer inducing
embrittlement and cracking. Temperature variations can threaten
the polymer layers. Especially in
constrained areas like bend stiffeners, buoyancy elements and
buried sections of the pipeline where
the temperature can reach above design limits. Anti-friction
tapes and high strength pressure tapes
are made from polyamides that degrade at high temperatures.
These layers can suffer from
embrittlement and disintegrate causing blocked annulus vent
tubes, but also soften and increase the
friction between steel layers and accelerate fatigue.
Design solutions
Temperature management is of paramount importance regarding
polymer layers. Gradually shut-
downs and start up with focus on slow thermal changes is
advised. Beware of chemical processes and
choose the polymer material that is best suited for the
operational environment for each
development.
4.2 Annulus environment
As the reported failures from CODAM and SureFlex shows annulus
environment represents a large
portion of failures of flexible risers. When flexible pipes
first was fabricated and installed it was a
common belief that the annulus would stay dry as long as the
outer sheath was intact. With this
assumption the fatigue life was calculated for dry environment.
After years of operation it was
realised that water and gas could diffuse through the internal
pressure sheath and condense inside
the annulus. The condensed water fills up the annulus and
degradation mechanisms act on both steel
and plastic layers. It was also discovered that the outer sheath
was relatively often damaged causing
the annulus to fill up with water and/or air, also increasing
the degradation of materials. The fatigue
life of the different layers is reduced and pipelines and risers
must be replaced before their expected
service life is over. Polymer failures due to exposure to water,
gas and air has been discussed in
section 4.1 with the most critical failures of cracking, end
fitting pull-out and collapsed carcass.
Corrosion of the armour wires is a severe consequence of flooded
annulus. Seawater ingress,
diffused H2S and CO2 are causes for corrosion of the armour
wires, however there is a large
knowledge gap regarding how corrosion acts and how severe the
individual cases of corrosion is. The
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24
corrosion rate of steel wires, the location of outer sheath
breach, composition of diffused gases, and
the combination of this are factors that need more research to
be fully understood. Pipelines and
risers have been in operation for several years with damaged
outer sheath and flooded annulus
without any serious degradation of the flexible pipe layers.
Design solutions
The best way to limit the failures caused by annulus environment
is QA/QC of fabrication and
installation and early detection of external breach. Also
monitoring vent gases from the annulus and
examining gas samples. Early detection will give the operators
time to repair or plan mitigating
measures.
Procedures have been developed for re-establishing blocked vent
systems, by drilling new vent ports
through end fitting, epoxy filling port or the external sheath
(ref. [24]).
4.3 End fittings
The end fitting is a complicated part of the flexible pipeline
system. This is the connection point
between the pipeline to a subsea module or riser to a production
facility. The end fittings comprise
the termination of all the layers in a flexible pipe, and needs
to seal off each layer to prevent leakage
into the annulus or out to the environment. Example of an end
fitting configuration is presented in
Figure 17.
Figure 17 - Example of end fitting configuration [2].
There are a number of failure mechanisms that can happen in the
end fitting. Carcass and pressure
sheath pull-out are discussed in the section of polymer related
failures. In addition there is an arising
problem with the termination of the tensile armour wires for
deep water risers where high tension
loads create challenges. Also the vent tubes for the annulus
vent system are normally installed
through the end fittings. These tubes may be blocked in
fabrication due to overbending, and also
from particles from dissolved polymer layers. Leakage through
the sealing inside the end fitting is a
reported problem that can cause annulus flooding or leakage to
the environment.
Design solutions
QA/QC in design and fabrication is important to assure the
integrity of the end fittings.
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25
5 Reported failures Reporting of failures is an important
process for further learning and information sharing between
manufacturers and operating companies. By sharing failure
experiences future failures can be
avoided and technology improvement can advance faster. This
chapter will present reported failures
from two sources; the CODAM database for failure reporting on
the NCS and the results from a JIP
lead by SureFlex including a number of international operating
companies.
Table 3 - Reported failure of flexible risers in the Norwegian
Continental Shelf (1995 2014). Source: CODAM, Ref.
Appendix C.
Table 3 presents the reported failures of flexible risers in the
NCS from 1995 to 2014. The data is
provided by the CODAM database which is managed by PSA - Norway.
The data used is listed in
Appendix C. There may be some discussion about how the failures
are categorised. In this table
collapse and burst are treated as different categories although
blocked or malfunctioning annulus
vent system may lead to both these failures. Also abrasion and
water filled annulus could be in the
same category, but some of the reported failures do not
coincide. It should be noted that the data in
CODAM is not always conclusive and some assumptions has been
made. The category other
includes: micro leakage, hydrate plug, lacking end termination
plug, dropped object, pigging and
disconnected anchor wire for mid water arch. A large part of
this chart is named unknown. This may
partly be because the reporting is insufficient from the
operators, but also the fact that flexible
pipeline integrity still is not 100% understood.
An obvious observation from table one is that collapse,
abrasion, water filled annulus and burst is the
most common failure modes. A significant number of these
failures happened due to blockage,
failure or faulty operation of the annulus vent system. As
discussed in the failure modes,
overpressure in the annulus can cause both burst and collapse.
Abrasion refers mainly to breach of
the external sheath, and it is known that poor procedures and
management of the fabrication and
installation process in the early years of flexible pipelines
are to blame. The positive note is that
these failure modes are well known, and relatively easy to do
something about. QA and QC
0
5
10
15
20
25
Major All
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26
throughout fabrication, storage, transportation, installation
and operation should be of paramount
interest for the operators when we see the outcome of
sloppiness.
Table 4 - Reported failure of flexible risers in Norway, UK and
worldwide [19].
Table 4 gives an overview of reported failures of flexible
risers worldwide and is a result of a JIP lead
by SureFlex (ref. [19]). This table is based on reports from
different institutes as well as information
provided from several operators worldwide through surveys. The
table is included in this thesis to
give an overview of failures not only in Norway, but also
worldwide. From this table we see that
external sheath damage and annulus vent system anomalies are
common failures also worldwide.
We also see that for the combined reporting from Norway and UK
polymer aging and pull-out
represent a large percentage of the total failures. This may be
caused by different reporting
processes, and more available data than for table 3, but
indicates that these failures are common.
However the percentage of the failures is allocated, all of them
should be of concern and preventive
measures should be taken. The category other includes: smooth
bore collapse, pigging damage,
upheaval buckling, excess torsion, excess tension, sheath
cracking and armour wire failure.
0,00%
5,00%
10,00%
15,00%
20,00%
25,00%
30,00%
35,00%
40,00%UKCS and NCS (2002) Worldwide (2010)
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27
6 Inspection techniques In order to inspect the flexible pipes
there are developed several inspection techniques for detecting
failures and degradations. An inspection is a time constrained
event that is planned with a beginning
and an end. Some inspection techniques require deployment of
vessels and equipment with experts
analysing the results. This chapter will explain both
conventional techniques that are used today as
well as new techniques that are under development. The
techniques will be presented with
theoretical background, offshore application and development
status.
The inspection techniques that will be described in this chapter
are:
1. Conventional inspection techniques
a. Visual inspection
b. Annulus pressure testing
c. Laser leak detection
d. Coupon testing
2. New inspection techniques
a. Radiography
b. Ultrasonic testing
c. Eddy current
d. Magnetic stress measurement
6.1 Conventional inspection techniques
6.1.1 Visual inspection
An inevitable method for inspecting pipelines and risers is
visual inspection. The most general
inspection method is the manual deck visual inspection to check
for obvious anomalies. Using divers
for shallow water inspection, Remotely Operated underwater
Vehicle (ROV) mounted cameras for
shallow and deep water and climbers for the above water part of
the riser will give a good
opportunity for close or general visual inspection.
Visual inspection should seek to identify the following
potential problems (ref. [2]):
1. Extent and type of marine growth
2. Pipe general integrity and condition, including leaks
3. Pipe outer sheath or external carcass integrity and
condition
4. Debris and dropped objects
5. Evidence of scour and estimated length of free spans
6. Condition of end fittings
7. Condition of cathodic protection system
8. Any identifiable damage, distortion, or degradation
9. Any identifiable disarrangement of pipe and disarrangement or
loss of pipe ancillary components
10. Interference with other subsea hardware
11. Loops and kinks
12. Movement or displacement from as installed conditions
13. Corrosion of and accumulation of marine growth in vent
valves
External general visual inspection
The external visual inspection is performed mainly to
investigate the integrity of the outer sheath. It
is well known that from the hang off point of the riser and down
to about 30m below sea level is the
most critical area for damaging of the outer sheath. Where the
riser is open (not inside bend
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28
restrictor or I-tube) regular intervals of visual inspection
using ROV or divers subsea or climbers
above sea level is common practice. For inspection and survey of
static flexible flowlines it is
common to deploy ROVs. There is however other solutions like
Autonomous Underwater Vehicles
(AUV) that can sit on the seabed for months and automatically
inspect the pipeline. [30] Explains the
operating principle and possible applications for their IMR AUV.
The AUV has a base subsea that is
connected to the host facility where it recharges between the
surveys. Here it can be manually
programmed to do a survey. When deploying the AUV automatically
performs the inspection or light
intervention tasks, returns to base and uploads the video and
data to a computer on the host facility.
This technology could potentially be a good alternative to ROVs.
AUVs has not been used for
pipeline inspection, but has the potential of carrying out this
type of work in the future.
External close visual inspection
If monitoring or inspection results raises the question of pipe
integrity, closer visual inspections of
the threatened area is performed in addition to the planned
insp